Migrating a Windows 10 installation from MBR to GPT/UEFI

I recently updated to Windows 10 on a Z170-based motherboard, but my Windows installation was an MBR-based one rather than a modern UEFI one. I wanted to switch to a UEFI installation for the marginally fast boot time – without losing data during the switch – but most of the information online about doing that involved either purchasing software to do it or dubious use of command-line utilities.

Somewhere, though, I came across the strategy of backing up the current Windows installation, making a clean one in UEFI mode in order to create the relevant boot partitions etc., and then restoring the Windows partition from the backup. I was upgrading my system drive to a larger SSD so it seemed like a good time to give it a go. And indeed, it worked fine, so I thought I'd document the steps I followed here in case it helps anyone else.

The process requires some spare space on another drive in order to save an image of your system drive. Naturally, if you follow these steps and something goes wrong, I don't take any responsibility – it goes without saying that you should make doubly-sure you have everything backed up.

The steps are roughly as follows:

1. Download the Windows 10 Media Creation Tool.
2. Use the tool to create a Windows 10 installation image on either a flash drive or a DVD. I used a flash drive.
3. Back up your system drive using drive imaging software. Macrium Reflect (free) should do, but I used the bootable version of Acronis True Image 2016 (not free) as I already had it. The software you use should allow you to selectively restore partitions from the backup to a partition of a GPT drive. If you are not using a bootable version of the imaging software, then it also needs to be able to restore backups to the system drive. Make sure you validate your backup to make sure there is nothing wrong with it.
4. At this point, I installed Windows 10 in UEFI mode to my new disk drive. It took minutes to install to a decent SSD from a decent USB 3.0 flash drive (surprisingly fast when compared to how long the upgrade from Windows 7 to 10 took). I disconnected all the other drives in my PC out of an abundance of caution. To install Windows in UEFI mode you need to boot from the Windows 10 installation media in UEFI mode. For me this was achieved by pressing F8 when on my BIOS splash screen and then selecting the entry for my flash drive that was prefixed with UEFI. If you're reusing your existing drive for the UEFI installation, you will need to delete all the partitions on it. You should be able to do this in Windows 10 setup, but I haven't tried that.
5. When prompted for a product key, select the option saying you don't have one, and then select the relevant Windows edition.
6. A couple of screens later, you'll need to select your disk drive and create a partition. If you booted the installer in UEFI mode correctly, another three partitions (recovery/system/MSR) should be automatically created when you create the one to install Windows to, leaving you with four in total. If that doesn't happen, you might not have booted in UEFI mode.
7. You can click through the remaining setup options – they don't matter much as we are just going to overwrite the Windows installation.
8. Once installation is complete, you need to restore the Windows partition from your backup to your drive using your drive imaging software. You need to only overwrite the current Windows partition with the one from the backup – take care not to restore the whole disk which will wipe your GPT layout.
9. Reboot and if everything worked as planned your old Windows installation should be back!
10. You should now be able to disable CSM in your BIOS, which should reduce POST time a bit.

Hopefully that helps someone else – doing this and disabling CSM did indeed make a reasonable improvement to my boot times.